Eldar Shafir of Princeton and Sendhil Mullainathan of Harvard have recently, with federal help, been exploring a third theory, that scarcity produces its own cognitive traits.

A quick question: What is the starting taxi fare in your city? If you are like most upper-middle-class people, you don't know. If you are like many struggling people, you do know.

Uh, most people don't live in places like NYC and DC where taking cabs is routine. Am I upper middle class or am I struggling or both? In any case, I don't know because I never take cabs on my own dime.

Poorer people have to think hard about a million things that affluent people don't. They have to make complicated trade-offs when buying a carton of milk: If I buy milk, I can't afford orange juice. They have to decide which utility not to pay.

These questions impose enormous cognitive demands. The brain has limited capacities. If you increase demands on one sort of question, it performs less well on other sorts of questions. ...

Shafir and Mullainathan have a book coming out next year, exploring how scarcity - whether of time, money or calories (while dieting) - affects your psychology.

Scarcity of time and money and the need to be thinking hard constantly due to those scarcities explains why African-Americans have traditionally watched so few hours of television per day. They just don't have time because they have to make all these careful decisions about what they can afford and what they can't. That's why you see so many black people all the time in public places hunched over a piece of paper with pencil in one hand and a calculator in the other.

Oh, wait, black Americans are traditionally the number one consumers of television according to decades of Nielsen numbers.

Obviously, scarcity is hugely important, but one big form of scarcity is scarcity of intelligence.